This allows to display objects that are in /ai/models/, but not managed
by the AI manager, and it follows fgfs' design principle that subsystems
should communicate over the property tree (if possible). This is a tad
slower, but the radar is only updated once every second.
Introduce a convenience member function in wxRadarBg for creating an
SGPropertyNode and initializing it in one step. Use this in
agRadar. This eliminates buggy behavior when the necessary radar
properties aren't defined.
